RioSerialLink: stop-and-wait command delivery (supersedes NAK-race resend)

Bench falsified the v1 retransmit (testlogs/riomash-patched-62500-retx):
resends tracked NAKs 1:1 (321/321) yet lamps still stuck/missed — under
mash bursts the NAK arrives after a newer command is already "latest",
so the wrong packet was resent; and total shreds never NAK at all.

Now commands are stop-and-wait: ONE in flight (_commandGate), resolved
by ACK, NAK, or AckTimeout (50ms); NAK/timeout retransmits THE SAME
packet up to CommandRetransmitLimit (2), then drops (idempotent - the
next state update supersedes). Attribution is exact by construction and
timeouts catch silent shreds. Control-byte replies bypass the gate so
board traffic is never delayed; a request's own reply (analog/version/
check) also resolves the wait, so request/reply exchanges never burn
the timeout even if the board sends no explicit ACK.

7 tests (same-packet resend, timeout retry+drop, ACK completion,
serialization, reply-resolves-request, stray-NAK no-op, disable);
282 green. Mash summary now reports NAK/timeout resends.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

tools/RioSerialMonitor/MashTest.cs

@@ -319,7 +319,7 @@ internal static class MashTest
         Raw($"firmware         : {(version is null ? "(no reply)" : version.ToString())}");
         Raw($"presses/releases : {presses} / {releases}  ({presses / mins:F0} presses/min)");
         Raw($"analog replies   : {analogReplies} (~{expectedPolls} poll slots; {100.0 * analogReplies / Math.Max(1, expectedPolls):F1}%), sentinels {sentinels}");
-        Raw($"framing / NAK    : {framing} / {naks} (NAK-triggered resends: {link.NakRetransmits})");
+        Raw($"framing / NAK    : {framing} / {naks} (command resends NAK/timeout: {link.Retransmits})");
         Raw("gap histogram    : " + string.Join("  ", bucketNames.Select((n, i) => $"{n}:{buckets[i]}")));
         Raw("longest gaps     : " + (longestGaps.Count == 0
             ? "(none)"